The objective of the program is to explore the temporal development of diabetic retinopathy and its possible precursors in terms of rheological and chemical parameters. The study will be accomplished by making a few supplemental measurements on the subjects that will be examined in the ongoing Early Treatment of Diabetic Retinopathy Study (ETDRS) investigation. The program will consist of two parts: a. Blood rheology tests will be made including rheoscopical data, sedimentation rate, viscosity measurements, and filter tests. b. A fluorophotometer will be used to obtain quantitative measurements of fluorescein leakage into the vitreous.